This invention relates to the construction of digital signal comparison devices, more particularly to a switching arrangement for such devices for controlling connections and disconnections of attenuator pads thereto. Devices of this nature may be used, for example, in telephone sets having no handsets, i.e., constructed for hands-free operation.
It is known, as illustrated in FIG. 1 to construct digital level comparison devices to have one input for the external level to be measured and another input for the nominal level which is in the form of a clock signal delivered by a clock pulse generator. The level comparison device has a counter range calibrated in equally large intervals. Each of the intervals of the counter range is assigned a special counter output leading to the digital control network which, when the counter reading is in its interval, produces a signal indicating the corresponding condition.
The foregoing circuit arrangement, as stated, is known in the art, more particularly for controlling connections and disconnections of attenuator pads to voice-grade channels, for example, in hands-free telephones. In the level comparison device, the counter range is graduated in separate equally large succeeding intervals, each interval corresponding to a 6 db transmission loss to be inserted. Since each interval of the counter range has an output, the line attenuation is changed by switching on or off a 6 db attenuator stage, when the counter reading is altered more than approximately 3 db, as viewed from the center of the interval. Not until an attenuation variation of about 3 db (calculated from the center of the section) is exceeded will another attenuator stage be switched on or off by a digital control network.
Such a circuit arrangement has a substantial disadvantage. To achieve a permanent loss condition in the voice-grade channels over a comparatively long period, a condition must be created where the measured external level equals the nominal level. As a rule, this can only occur in an analog system, since in this case a continuous change of the inserted channel loss is possible. In a digital system, the insertion will always occur incrementally, and in this way the attenuation in the voice-grade channels will change in fixed specified steps. That means that the exact decrement which would be needed for matching the external level to the nominal level by varying the attenuation can theoretically be adjusted in very specific cases only. In undertaking such adjustment, the attenuation being adjusted will initially be larger than that needed to achieve external levels that are equal to nominal levels. Thus, the next step will be to reduce the attenuation, and as a result the voice-grade channel attenuation is again too low. Hence, it follows that particularly with constant levels (signals on the circuit), an attenuator stage is constantly added to or removed from the circuit, which produces disturbing results in the case of 6 db attenuator stages.
An obvious solution to the foregoing problem consists in making the attenuator stage so small that adding or subtracting stages is no longer felt as disturbing by the human ear. To accomplish this, attenuator stages having values no greater than 2 db should be chosen. However, this would mean that with a damping range of, e.g., 48 db, 24 stages would be required. The undesirability of such complexity is obvious. Since the volume range of the human voice has a variation range of about 4 db, such a control device would control over the volume range of the voice. This is absolutely undesirable, however, and for this reason alone this approach does not seem practicable.
An object of the invention is, therefore, to provide a switching arrangement for a level comparison device which regulates at sufficiently large level intervals so as not to have regulation over the volume range of the human voice and which nevertheless, after adjustment to a mean volume level, prevents hunting between two attenuator stages.